[quote author="s3c"]The reverse LED is still interesting though, before replacing it I looked at it through my camera and it did light up dimly, could it be that some of the LEDs are in fact correct but the R4 mishap made it appear otherwise?[/quote]
(stealing this comment aside for a new topic)
We know that a regular non-light-emitting diode will conduct slightly under reverse bias if a larger threshold is exceeded, and that's what a Zener is, but what I don't know is whether an LED will emit photons when reverse-biased. I haven't looked at the specs for the part in question, but perhaps it documents the reverse voltage and reverse current.
P.S. What is that part number?
It should be a SFH-480
Only for the Curious:
The SFH-480 has a 5V reverse voltage, with 10 nA reverse current, so it at least seems plausible that the backwards LED could still light up. It probably wouldn't work reliably if you actually wanted it to operate reverse biased, but maybe there's enough of a trickle of current for a dimly lit reverse LED.
I'm not sure if it was lighting up or not. Since fixing the LED I've made several attempts (short of removing the LED again) at trying to get the thing to fail the self test with no success. Bright sunny rooms, covering the LED and receiver with everything I can think of to block out the light, and every time it passes. I can't see anything in the code that would lead it to pass, but I'm not very good at debugging code.
Thanks for the update.
I thought you said that your camera showed a dim output from the IR LED? I didn't know about that trick until you mentioned it, and then I tried it with a regular TV remote and saw an obvious white LED shape flickering. I've already changed my USB IR Toy by resoldering, so I can't try the reverse with my camera. I was only curious, though, since I never thought about whether an LED would light up with reverse current.
On that note, most scanning LED matrix circuits have some amount of dim ghosting, and I wonder if this could be due to reverse current. I suppose I should test one of those circuits to find out whether it's just timing.
Seeed also inspected each unit with a camera and saw it light. I made it fail the self-test by encasing the receiver (or transmitter) in "blu-tack" sticky putty.
Now that I think about it I had a problem like this in a previous project, when switching a load with a uC and transistor you need to set the pin to highz to turn it off since setting it as an output on low doesn't give you a zero voltage but something a little above turning it on slightly, luckily this can be fixed in software.
What you describe can happen sometimes, but it's highly dependent upon the circuit that you have attached, and somewhat upon the type of port circuitry. A low on most PIC ports will be no higher than 0.4 V, which is typically too little to turn on any LED. Also, some circuits will go high if you use High-Z, certainly if there is any kind of pull-up. The moral is that you need to know what will happen with your particular circuit and choose HiZ, high, or low as appropriate.
Well with the Irtoy I'm pretty sure that will fix the always partially on bug. Go look at the ir led after the tvbgone firmware has vinished for example, it's still partially lit.